unusual hearing issue -- what is this?

several decades ago, Prof. Don Campbell showed me that I have an unusual hearing condition as follows -- tap a tuning fork and listen to the tone. Then touch the base of the still vibrating tuning fork to the mastoid bone -- for me the tone changes significantly when this is done. Don commented that this rare condition is why I always sing off key -- what I hear from outside via my ears is NOT the tones I [try to] reproduce with my voice -- my hearing issue automatically corrects my singing to be off key. I've tried online but can not find a description of this condition -- what is this? [I don't think there is any treatment other than not singing.] {I can hear just fine -- but, apparently the tones are all "wrong".}

thanks for any lead or link on this.